1. Field of Invention
The present invention relates to a silica glass article and a manufacturing process therefor. More particularly, the invention relates to a silica glass article including an optical fiber suitable for use in an ultraviolet region, having an excellent initial transmission characteristic and capable of preventing increase in a transmission loss occurring due to ultraviolet ray irradiation, and to a manufacturing process therefor. The industrial utility of ultraviolet rays having a wavelength of 160 nm to 300 nm has increased in industrial fields of photolithography, a laser process, sterilization, disinfection and the like. A silica glass article according to the present invention is free from substantial deterioration caused by ultraviolet ray irradiation and can be advantageously be used in such fields.
2. Description of Related Art
Silica glass articles have been used as light transmitting mediums, such as optical fibers and various optical elements. In particular, use of the optical fibers having advantages of light weight, small diameter and no induction, has recently been widened in various industrial fields including communication, image transmission and energy transmission. As one of the fields, use of the optical fiber to transmit ultraviolet rays has been expected in the medical and precise processing fields. However, when glass is irradiated with ultraviolet rays, it deteriorates and its transmission loss increases. That is, there arises a problem in that deterioration takes place because of ultraviolet ray irradiation. Since the transmission loss of a silica optical fiber having the silica glass as the core thereof is smaller than that of an optical fiber made of multicomponent type glass, the silica optical fiber is a preferred element to transmit ultraviolet rays. However, the problem of the deterioration which takes place because of ultraviolet ray irradiation remain unsolved.
It is possible for light transmission in silica glass to be superior to that in air if the wavelength is not longer than 200 nm. The reason for this is that dissociation of oxygen gas takes place because of ultraviolet ray irradiation in air. Therefore, a high transmission can be expected if the deterioration which takes place because of ultraviolet ray irradiation can be reduced in the wavelength region not longer than 200 nm.
It has been considered that the deterioration which takes place because of ultraviolet ray irradiation is mainly attributed to a defect in glass. In the present invention, the xe2x80x9cdefect in glassxe2x80x9d means a broken portion of the glass network structure or a portion of the glass network structure that is stretched due to a distortion of the glass and is apt to break easily. FIG. 4 shows a plurality of examples of reported defects in glass of silica glass. As representative defects in glass, defects related to Exe2x80x2 center (xe2x89xa1Sixe2x80xa2) and oxygen-deficient type defects (xe2x89xa1Sixe2x80x94Sixe2x89xa1) are exemplified. The above-mentioned defects in glass absorb ultraviolet rays at wavelengths of 163 nm, 215 nm and 245 nm. It has been considered that the foregoing defects in glass occur in a glass synthesized in an atmosphere somewhat lacking in oxygen, or in a glass having a low concentration of OH groups.
As a technique for reducing deterioration due to ultraviolet ray irradiation of silica glass, a technique has been disclosed in JP-A-5-147966 (hereinafter referred to as Document (1)) (The term xe2x80x9cJP-Axe2x80x9d used herein means an unexamined published Japanese patent application), in which the content of OH groups in a pure silica core is adjusted to from 10 ppm to 1000 ppm, the contents of F (fluorine) is adjusted to from 50 ppm to 5000 ppm and the contents of Cl(chlorine) is adjusted to substantially zero. An optical fiber thus obtained has an excellent initial characteristic of transmitting ultraviolet rays and is capable of reducing deterioration due to ultraviolet ray irradiation because fluorine is contained in a specific amount.
There are several known techniques that are not aimed to improve deterioration due to ultraviolet ray irradiation but are related to an improvement in radiation resistance of a fiber for transmitting visible rays or near infrared rays. For example, JP-A-60-90853 (hereinafter referred to as xe2x80x9cDocument (2)), has suggested a process in which any one of a glass soot body, a transparent glass preform and an optical fiber is processed in a hydrogen atmosphere to delete defects in the glass so as to improve the radiation resistance of the optical fiber. In the foregoing document, only a result of measurement of an increase in the loss experienced with respect to a near infrared rays having a wavelength of 1.3 xcexcm is described. In addition, the effect of improving the ultraviolet ray resistance obtained by the above-mentioned process disappears within about two months.
In xe2x80x9cImprovement in Radiation Resistance of Optical Fiber by Hydrogen Treatment and xcex3-Ray Irradiationxe2x80x9d, Tomon, Nagasawa, et al. pp. 1-213, Vol. 1, papers for lectures in National Conference of Semiconductor and Its Material Section of Electronic Communication Society, 1985, issued in 1985 by Electronic Communication Society (hereinafter referred to as xe2x80x9cDocument (3)xe2x80x9d), a process has been reported for the purpose of preventing increase in light absorption of a pure silica-core optical fiber at a wavelength of 630 nm (visible ray) occurring due to xcex3-ray irradiation. In this document, two-step treatment for an optical fiber is performed. In the first step, an optical fiber is doped with hydrogen molecules and then, in the second step, is irradiated with xcex3-rays. Thus, seeds (precursors) of defects in the glass are converted into defects that absorb photon energy of a 2 eV band. Then, hydrogen previously dispersed in the fiber in the previous step and the defects in glass are chemically bonded to each other so as to improve the radiation resistance in the visible ray region. Also in the Document (3), there no description about the characteristic of the fiber against ultraviolet rays.
U.S. Pat. No. 5,574,820 (hereinafter referred to as xe2x80x9cDocument (4)xe2x80x9d), suggests an optical fiber and its manufacturing process that serves as a means for preventing increase in a loss in a visible ray region when a pure silica core fiber is used as an image fiber for transmitting visible rays in a radiation field. The proposed optical fiber is manufactured by previously irradiating pure silica core fiber with radiation in a large dose of 105 Gy or greater, so that increase in the loss in a visible ray region having a wavelength of from 400 nm to 700 nm does not exceed 30 dB/km. Moreover, a process for manufacturing the optical fiber has been suggested, but the characteristic in the ultraviolet ray region has not been described.
JP-A-5-288942 (hereinafter referred to as xe2x80x9cDocument (5)), as in Document (4), has suggested a process for improving radiation resistance of an image fiber for transmitting visible rays. In the process, an image fiber is irradiated with g-rays in a large dose of 107 Roentgen to 109 Roentgen (105 Gy to 107 Gy) and then is heated in a hydrogen atmosphere. Also no description about the characteristic in the ultraviolet ray region has been made in the above-mentioned document.
In the Document (2), hydrogen is added so that the radioactive resistance of the optical fiber in the near infrared rays is improved. Recently there have been disclosed several processes in which hydrogen molecules are added to silica glass in an attempt to improve ultraviolet ray resistance. For example, JP-A-3-23236 (hereinafter referred to as xe2x80x9cDocument (6)xe2x80x9d) suggests silica glass in which OH groups are contained in an amount of 100 ppm or higher, substantially no oxygen defect exists and hydrogen gas is contained, so that ultraviolet ray resistance is improved. JP-A-5-32432 (hereinafter referred to as xe2x80x9cDocument (7)xe2x80x9d) suggests a process, in which deterioration due to ultraviolet ray irradiation is prevented by controlling the concentration of hydrogen in silica glass to 1.5xc3x971017 molecules/cm3 or higher. Moreover, the concentration of chlorine is made to be 100 ppm or lower to reduce hydrogen consumption in glass when ultraviolet ray irradiation is performed so as to maintain ultraviolet ray resistance. JP-A-6-16449 (hereinafter referred to as xe2x80x9cDocument (8)xe2x80x9d) suggests silica glass which has improved ultraviolet ray resistance by designing to contain OH group in an amount of 100 ppm or lower and chlorine in an amount of 200 ppm or lower, and to have a hydrogen concentration of 1016 molecules/cm3 or lower, a refractive index fluctuation of 5xc3x9710xe2x88x926 or lower and a birefringence of 5 nm/cm or lower. U.S. Pat. No. 5,668,067 (hereinafter referred to as xe2x80x9cDocument (9)xe2x80x9d) suggests silica glass in which the amount of OH groups is 50 ppm or smaller and hydrogen is contained by at least 1018 molecules/cm3 and which is free from optical damage if the silica glass is irradiated with 107 pulses of KrF laser, the output of which is 350 mJ/cm2. U.S. Pat. No. 5,679,125 (hereinafter referred to xe2x80x9cDocument (10)xe2x80x9d) suggests silica glass which has improved ultraviolet ray resistance because hydrogen molecules are added to silica glass to which fluorine has been added.
JP-A-7-300325 (hereinafter referred to as xe2x80x9cDocument (11)xe2x80x9d) suggests a process which is able to improve ultraviolet ray resistance by means similar to that suggested in Document (5) in such a manner that hydrogen-molecule-contained silica glass is irradiated with xcex3-rays so as to make the concentration of hydrogen in the irradiated silica glass to be 5xc3x971016 molecules/cm3 or higher so that the ultraviolet ray resistance is improved. JP-A-9-124337 (hereinafter referred to as xe2x80x9cDocument (12)xe2x80x9d) suggests a process with which the ultraviolet ray resistance is improved by irradiating glass containing hydrogen molecules at a concentration of from 2xc3x971017 molecules/cm3 to 5xc3x971019 molecules/cm3 with ultraviolet rays of 150 nm to 300 nm for 20 hours or longer.
The Document (1) discloses an optical fiber having an excellent initial transmission characteristic of ultraviolet rays. However, a satisfactory effect cannot be obtained to prevent the deterioration due to ultraviolet ray irradiation. On the contrary, absorption caused at the absorption edge of ultraviolet rays is enlarged undesirably. Therefore, adjustment of an optimum amount of addition cannot easily be achieved.
No description has been made about the deterioration due to ultraviolet ray irradiation in each of the Documents (2) to (5) relating to improvement in the radiation resistance required for transmitting near infrared rays. As described later, the processes adapted to a fiber for transmitting visible rays or a fiber for transmitting near infrared rays cannot maintain the effect to prevent deterioration due to ultraviolet ray irradiation for a required period of time. Moreover, unsuitable means for an optical fiber for transmitting ultraviolet rays have been employed.
The processes disclosed in the Documents (6) to (11) are arranged in such a manner that the contents of OH groups, F or Cl are adjusted. Although the above-mentioned adjustment of the components attains an effect of initial defects in glass, a satisfactory effect cannot be attained to reduce defects induced by ultraviolet rays.
The hydrogen treatment employed in the processes disclosed in Documents (6) to (12) is such that the defects in glass caused by ultraviolet ray irradiation and the hydrogen molecules dispersed in the glass by the hydrogen treatment are bonded to each other so that increase in absorption of light is restrained. The restraining period, however, is limited to a period of time in which hydrogen molecules remain in the glass. Since the processes disclosed in the Documents (6) to (12) are mainly adapted to a bulk-form glass member, the volume of the glass member is sufficiently large with respect to the velocity at which hydrogen in the glass is dispersed. It is considered that hydrogen molecules remain in the member for a long time and thus ultraviolet ray resistance can be maintained.
If the techniques in the Documents (6) to (12) are adapted to an optical fiber, hydrogen is undesirably dispersed out the outside in a short time. Thus, there arises a problem in that the ultraviolet ray resistance cannot be maintained. That is, hydrogen molecules in an optical fiber (having an outer diameter of 125 mm) are generally gradually discharged to the outside of the optical fiber at a room temperature and the concentration is lowered to about {fraction (1/10000)} in about two months as shown in FIG. 6. That is, the above-mentioned restraining effect is effective in only about two months after the hydrogen treatment has been performed. Therefore, increase in the absorption cannot be restrained for a long time with the conventional techniques.
In view of the foregoing, an object of the present invention is to provide a silica glass article for use as an optical fiber or a bundle fiber for transmitting ultraviolet rays, having an excellent transmission characteristic of ultraviolet rays and exhibiting a satisfactory resistance against deterioration due to ultraviolet ray irradiation.
Another object of the present invention is to provide a silica glass article substantially free from considerable deterioration even if irradiated with ultraviolet rays having a wavelength not longer than 200 nm and having superior light transmission as compared to that in air.
Another object of the present invention is to provide a silica glass fiber having a sufficient resistance against deterioration due to ultraviolet ray irradiation and to provide a method of producing the same without damaging its coating.
Still another object of the present invention is to provide a silica glass article having excellent resistance against deterioration due to ultraviolet ray irradiation.
A still further object of the present invention is to provide a manufacturing process for the foregoing silica glass articles.
Other objects and effects of the present invention will become apparent from the following description.
The above described objects of the present invention have been achieved by providing a process of manufacturing a silica glass article comprising the steps of:
(1) irradiating a silica glass article with electromagnetic waves to generate defects therein; and
(2) immersing the thus irradiated silica glass article in an atmosphere comprising a hydrogen gas, thereby providing the resulting silica glass article with a characteristic that is effective for preventing it substantially from increasing its absorption within an ultraviolet region due to ultraviolet ray irradiation.
The electromagnetic waves can be ultraviolet rays, vacuum ultraviolet rays, X rays or xcex3 rays having a photon energy of not less than 3.5 eV with which defects in glass can be generated.
An advantageous dose of the electromagnetic waves for irradiation is from 10 Gy to 104 Gy.
Step (2) is advantageously carried out when the partial pressure of the hydrogen gas is from 0.5 atm to 10 atm and the temperature is not lower than room temperature.
The process can include the additional step of (3) irradiating again the silica glass article that has been subjected to the step (2), with electromagnetic waves while hydrogen molecules remain therein.
It is advantageous for the silica glass article to contain hydrogen molecules at a concentration of not lower than 1xc3x971016 molecules/cm at the beginning of the step (3).
After the step (3), an additional step (4) can be carried out for reducing the hydrogen molecules that are remaining in the silica glass article to not higher than 1xc3x971016 molecules/cm3.
The above-described process of manufacturing a silica glass article is advantageously applied to manufacturing an optical fiber. Step (2) is advantageously carried out at a partial pressure of the hydrogen gas of from 0.5 atm to 10 atm and at a temperature in a range from room temperature to a highest temperature with which coating on the optical fiber is not damaged. Step (2) should be carried out at a temperature of from 80xc2x0 C. to 200xc2x0 C.
The optical fiber can be a bundle fiber formed by bundling a plurality of optical fibers or an optical fiber for a bundle fiber before wound.
According to another aspect of the invention, there is provided a silica glass article which is manufactured by a process comprising the steps of:
(1) irradiating a silica glass article with electromagnetic waves to generate defects therein; and
(2) immersing the thus irradiated silica glass article in an atmosphere comprising a hydrogen gas, thereby providing the resulting silica glass article with a characteristic that is effective for preventing it substantially from increasing its absorption within an ultraviolet region due to ultraviolet ray irradiation;
The process can include a further step of (3) irradiating again the silica glass article that has been subjected to the step (2), with electromagnetic waves while hydrogen molecules remain therein.
The process can further include after the step (3), a step of (4) reducing the hydrogen molecules that are remaining in the silica glass article to not higher than 1xc3x971016 molecules/cm3 by allowing to stand in the atmosphere or by heating at 80xc2x0 C. or lower.
This invention also provides an optical fiber comprising a core and a clad having a refractive index lower than that of the core, the optical fiber having the characteristic that when 108 pulses of KrF excimer laser having a wavelength of 248 nm are applied to the optical fiber, as ultraviolet ray irradiation for an evaluation of ultraviolet resistance, at an output of 10 mJ/cm2 and a pulse frequency of 100 Hz, the optical fiber as measured with a sample of one meter length has a transmittance of not less than 90% of the transmittance measured prior to the irradiation in a wavelength region of ultraviolet ray from 160 nm to 300 nm.
The process can further include a step of (3) irradiating again the optical fiber that has been subjected to the step (2), with electromagnetic waves while the hydrogen molecules that are remaining in the optical fiber is not lower than 1xc3x971016 molecules/cm3, so as to impart the characteristic that when 108 pulses of KrF excimer laser having a wavelength of 248 nm are applied to the optical fiber, as ultraviolet ray irradiation for an evaluation of ultraviolet resistance, at an output of 10 mJ/cm2 and a pulse frequency of 100 Hz, the optical fiber as measured with a sample of one meter length has a transmittance of not less than 90% of the transmittance measured prior to the irradiation at a wavelength of 248 nm.
The process can further include after the step (3), a step of (4) reducing the hydrogen molecules that are remaining in the optical fiber to not higher than 1xc3x971016 molecules/cm3 by allowing to stand in the atmosphere or by heating at 80xc2x0 C. or lower. The core of the optical fiber can include high-purity silica glass containing fluorine. The core of the optical fiber can include high-purity silica glass which contains OH groups in an amount of not less than 100 ppm and which does not contain more than 1 ppm of C1. The high-purity silica glass advantageously further contains OH groups in an amount less than 100 ppm. The optical fiber can be a bundle fiber formed by bundling a plurality of glass fibers.
That is, according to the present invention, there can be provided an optical fiber having the characteristic that when 108 pulses of KrF excimer laser having a wavelength of 248 nm are applied to the optical fiber, as ultraviolet ray irradiation for an evaluation of ultraviolet resistance, at an output of 10 mJ/cm2 and a pulse frequency of 100 Hz, the optical fiber as measured with a sample of one meter length has a transmittance of not less than 90% of the transmittance measured prior to the irradiation in a wavelength region of ultraviolet ray from 160 nm to 300 nm.
Furthermore, according to the present invention, there can be provided an optical fiber having the characteristic that when 108 pulses of KrF excimer laser having a wavelength of 248 nm are applied to the optical fiber, as ultraviolet ray irradiation for an evaluation of ultraviolet resistance, at an output of 10 mJ/cm2 and a pulse frequency of 100 Hz, the optical fiber as measured with a sample of one meter length has a transmittance of not less than 90% of the transmittance measured prior to the irradiation at a wavelength of 248 nm.
In a preferred embodiment, the electromagnetic waves for use in step (3) is KrF or ArF excimer laser. When the KrF laser beam is used in step (3), the irradiation is preferably performed in such a manner that 106 to 107 pulses are applied at 1 mJ/cm2/pulse to 200 mJ/cm2/pulse. Moreover, step (4) is performed in such a manner that the silica glass article is heated at a temperature from room temperature to 80xc2x0 C.
In another preferred embodiment of the present invention, the optical fiber comprises a core comprising a high purity silica glass, and a clad comprising a high purity silica glass containing fluorine. In addition, the optical fiber according to the present invention preferably comprises a core comprising a high purity silica glass and containing substantially no dopant for adjusting refractive index.